CAMBRIDGE DICTIONARY: We can use 'own' as a pronoun after a possessive determiner (e.g. my, his, their). We often use it in the pattern noun + of + possessive determiner + own:
eg 'Did you have a flat- of your own- when you were a student, or did you share?'
THE ABOVE EXAMPLE USES POSSESSIVE DETERMINER/ADJECTIVE 'YOUR'. HOW DOES THIS MAKE 'OWN' A PRONOUN ? HOW DOES 'OWN' EXPRESS A NOUN? Thank you.

I'm afraid we don't comment on what other grammars or books say, but I can confirm that 'own' can indeed act as a pronoun in some cases. For example, imagine you offer to lend me your car to go to work, since mine isn't working very well. I could say, 'No, thanks. I'll go in my own.'

Hi
Please help me with the following .
He says his surname is Smith . What is your surname .
In the above sentence his surname and your surname , which word is the possessive adjective and possessive pronoun.
Thank you